Lorenz Cipher Chi Wheel Demonstrator (Simplified SZ42 Teaching Model)

Turn a letter into cipher with your own hands, the way the Lorenz machine did it: five wheels of pins, a press of a lever, and a window that shows you the answer.

What it is

This is a simplified teaching model of the Lorenz SZ42 cipher machine, the teleprinter cipher that Bletchley Park called Tunny. It has the five chi wheels only, with their real cam counts of 41, 31, 29, 26 and 23. The psi and motor wheels of the real machine are not included, so this is not a full SZ42 and it will not reproduce real wartime traffic. What it does show, clearly and mechanically, is the core idea: each of the five bits of a teleprinter character is added (XOR) to a bit from its own chi wheel, and then every wheel steps on.

Every cam is a push pin. Pin out is 1, pin in is 0, so you set your own key. A follower rides on each wheel, and a little sliding strip in each of the five lanes works as a mechanical XOR gate. Set the plaintext bits with five slider tabs, read the cipher bits in five windows, press the lever, and all five wheels move on by exactly one cam. Deciphering is the same process with the same start positions.

It pairs with the free Lorenz game at dexmlabs.app/lorenz/, which has a chi only mode that follows exactly the same rules, so you can check your work on screen.

A little history: the Lorenz machine carried messages for the German High Command. Bill Tutte worked out its logical structure in 1942 from intercepted messages without ever seeing one, and the Colossus computers at Bletchley Park were built to help find the chi wheel settings. This model is my own simplified design for teaching and is not affiliated with any museum.

What you get

• ✔ 5 chi wheels (41, 31, 29, 26 and 23 cams) with start numbers on every rim

• ✔ 160 push pins on 5 sprues (150 needed, 10 spares)

• ✔ Wheels in three pin hole tolerances, plus a pin fit coupon to choose between them

• ✔ Optional fixed key wheels with a documented example key moulded in, so no pins are needed

• ✔ Step lever, pawl bar and printed detent springs: one press steps every wheel by one cam

• ✔ 5 followers, 5 plaintext sliders and 5 XOR strips with windows

• ✔ Engraved ITA2 card (the teleprinter alphabet) that sits in a pocket on the base

• ✔ README with print settings, assembly and a worked example

• ✔ CONVENTIONS.md, SOURCES.md and a plain Python reference script with test vectors

• ✔ Parametric Python source, so you can rebuild the pin wheels at any hole clearance

What you need

• About 280 g of PLA (estimate) in up to three colours

• 4 M3 x 16 countersunk screws

• One 5 mm round rod, 100 mm long, or print the included axle

• A small file or knife to clean the pins

Print settings

• PLA or PETG, 0.4 mm nozzle, 0.2 mm layers

• 3 walls, 4 top and bottom layers, 15% infill

• No supports. Every file is already in its print orientation.

• Print the pin fit coupon first, in the same filament as the wheels

• Pins: slow outer walls help them come out straight

• Every part fits a 220 x 220 mm bed. Estimated total print time is about 18 hours.

Assembly

1. Print the coupon, try the test pins and choose the wheel set with the fit you like.

2. Snap the pins off their sprues and push them into the wheels.

3. Screw the left side plate to the base, lay the pawl bar in place, then fit and screw the right side plate.

4. Hold the wheels and the step lever between the plates and push the 5 mm rod through.

5. Fit the five followers and push the printed pivot rod through.

6. Thread the XOR strips into the display panel from below and drop the panel into place.

7. Push the plaintext sliders in through the panel slots until they click into their notches.

8. Put the ITA2 card in its pocket. Full step by step notes are in the README.

How to encipher a letter

1. Set a key: pins out for 1, pins in for 0 (or use the fixed key wheels).

2. Roll each wheel towards you until its start number sits under the pointer, for example 1 1 1 1 1.

3. Find your letter on the ITA2 card. H is 0 0 1 0 1.

4. Push in the slider tabs for every 1 and leave the 0s out.

5. Read the five windows from left to right and look the result up on the card. With the example key at 1 1 1 1 1, H comes out as V.

6. Press the lever down and lift it back. Every wheel moves on one cam, ready for the next letter.

To decipher, set the same start positions and put the cipher letters in. The plain text comes back out.

Please note: these files have been checked carefully in software for fit and clearances, but I have not made a test print of this version, and the photos are renders. The pawls and detents are small printed springs, and the lever is returned by hand.

If you print one, I'd love to see it. Post a make with a photo, good or bad, and it helps me improve the next version.

Happy to tweak the design on request.